Methods and apparatus for improving resolution and reducing noise in an image detector for an electron microscope

ABSTRACT

Methods and apparatus are provided which improve the performance of electron imaging detectors by reducing the total interaction volume of the detector and/or reducing the energy converting volume of the detector. In one embodiment, a method for improving resolution and reducing noise in an imaging electron detector for an electron microscope is provided and includes the step of decelerating the energetic electrons either before the electrons interact with, or as the electrons interact with, the energy converting volume of an imaging electron detector. In other embodiments, the lateral spatial travel of energetic electrons is limited as they traverse the imaging electron detector, or, the extent of electron back scatter from the energetic electrons is limited.

CROSS REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of U.S. ProvisionalApplication Serial No. 60/049,397, filed Jun. 13, 1997.

BACKGROUND OF THE INVENTION

The present invention relates to methods and apparatus for use in anelectron microscope to improve the detection of electron images, andmore particularly to methods and apparatus for converting and using theenergy of the electrons in the image to improve image resolution andreduce noise.

Electron microscopes use a beam of accelerated electrons which passthrough or are deflected by a sample to provide an electron image and/ordiffraction pattern of the sample. To provide a record of these imagesand/or diffraction patterns, at least a portion of the kinetic energy ofsuch electrons is converted into some other form of energy which can bemeasured and permanently stored. One example of such an energyconversion process is the excitation of silver halide grains in aphotographic emulsion. Chemical development converts the grains into apermanent distribution of silver particles, the density of which can bemeasured by commercially-available microdensitometers.

Another example of an energy conversion process for the electrons is thegeneration of light images by impinging the electrons onto scintillatormaterials (e.g., phosphors), and then capturing the light images and/orpatterns onto a two-dimensional imaging sensor. One example of such animaging sensor is a charge coupled device (CCD). The output from the CCDmay be read as an analog signal, measured by an analog to digitalconverter, and then displayed (such as on a video monitor) and/or storedpermanently (such as in the memory of a computer).

These two examples employ different means to convert and store therelative intensities of the electrons. However, the process of thedeposition of the electrons' energies is the same. That is, once anaccelerated electron enters the solid volume of the detector(photographic film emulsion or scintillator film), it starts to loseenergy to the solid. This energy loss is at a rate which depends on theinitial energy of the electron and the solid material through which itis traveling. The electron is also scattered randomly by the fieldssurrounding the atoms of the detector in a manner which alters theelectron's direction or path of travel.

The result is that a series of accelerated electrons of the same initialenergy, entering the solid detector at a specific point, will generate aset of paths which together fill a region of space resembling a cloud.This cloud-shaped volume can be defined as the envelope of all possiblepaths and is termed the interaction volume of the electron beam in thedetector. The energy of the electron beam and the average atomic numberdensity (Z density) of the detector material together determine theelectron path's average behavior and thus the size and shape of theinteraction volume.

Higher electron energies cause the interaction volume to be larger,while denser materials in the detector will cause it to be smaller.Denser materials also increase the average deflection angles ofelectrons and therefore cause more scattering of electrons back out ofthe detector. The calculation of paths the electrons will take and theirresulting statistics is known in the art as “Monte Carlo” simulation.

The interaction of high energy electrons with the volume of the solidmaterial of the detector generates spreading and noise which constituteprimary limitations on the amount of spatial and intensity informationobtainable from the incident electron image. One approach to dealingwith the non-zero interaction volume of the detector has been to makethe detector as thin as possible. In a thin sheet of film, for example,a beam of accelerated electrons experiences minimal scattering beforeexiting.

However, a disadvantage of using a thin film of the detector material isthat only a small fraction of each electron's energy is utilized. Makingthe detector thicker increases sensitivity, but also increasesscattering and degrades resolution. Further, where a scintillator isoptically coupled to a CCD (such as, for example, by a fiber optic), thescintillator can be made thin so that light is generated only in a smallvolume near the point of entrance of the electrons. However, theelectrons continue to be scattered after leaving the scintillator, withsome electrons being back-scattered into the scintillator again.

Such back-scattered electrons will cause scintillation as well, creatingan extended, noisy flare around the central spot of light created by theelectrons in their incoming traversal of the scintillator. One solutionto the problem of back-scattering is taught by Mooney et al, U.S. Pat.No. 5,635,720. There, the fiber optic which couples the scintillator tothe detector is replaced by a light metal fold mirror and a lenscoupling. Thus, the number of back-scattered electrons reentering thescintillator is reduced, but at the cost of a decrease in lightgathering efficiency compared to fiber optics which decreases theoverall sensitivity of the camera.

Thus, the need remains in this art for a method and apparatus forreducing the contribution to the total noise and resolution loss causedby the initial step of electron interaction with the detector withoutsacrificing sensitivity in light collection.

SUMMARY OF THE INVENTION

The present invention addresses that need by providing methods andapparatus which improve the performance of electron imaging detectors byreducing the total interaction volume of the detector and/or reducingthe energy converting volume of the detector. In accordance with oneaspect of the present invention, a method for improving resolution andreducing noise in an imaging electron detector for an electronmicroscope is provided and includes the step of decelerating theenergetic electrons either before the electrons interact with, or as theelectrons interact with, the energy converting volume of an imagingelectron detector. By “energy converting volume”, it is meant theinteraction volume of the electrons within the energy converting mediumof the detector. By “interaction volume”, it is meant a region of spacewhich is the envelope of all possible paths of the electrons in thevolume of the detector.

In accordance with another aspect of the present invention, a method forimproving resolution and reducing noise in an image detector for anelectron microscope is provided and includes the step of limiting thelateral spatial travel of energetic electrons as they traverse theimaging electron detector. In accordance with yet another aspect of thepresent invention, a method for reducing noise in an image detector foran electron microscope is provided and includes the step of limiting theextent of electron back scatter into the imaging electron detector. Inaccordance with yet another aspect of the present invention, a methodfor improving resolution and reducing noise in an image detector for anelectron microscope is provided and includes the step of selecting apixel size for the measuring and/or storing portion of the imagingelectron detector which encompasses substantially the entire interactionvolume for a majority of energetic electrons in the detector.

The process of the present invention has utility in transmissionelectron microscope (TEM) analysis. Accordingly, it is a feature of thepresent invention to provide a method of detecting and imaging energeticelectrons which not only enhances resolution, but also reduces noise.This and other features and advantages of the invention will becomeapparent from the following detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a typical arrangement forimage detection in a transmission electron microscope;

FIG. 2 is an illustration, in the form of a geometrically-orientedschematic, showing the arrangement of the elements of the presentinvention in a typical transmission electron microscope (TEM);

FIG. 3 is an illustration, in the form of a geometrically-orientedschematic, showing the arrangement of the elements of the presentinvention in a typical transmission electron microscope operating in ascanning mode (STEM);

FIG. 4 is an illustration, in the form of a geometrically-orientedschematic, showing the arrangement of the elements of the presentinvention in a typical scanning electron microscope (SEM);

FIG. 5 is a schematic illustration of another embodiment of theinvention using a decelerating coating;

FIG. 6 is a schematic illustration of another embodiment of theinvention in which areas of scintillator materials between walls ofnon-scintillator materials are used;

FIGS. 7A-7C are schematic illustrations of another embodiment of theinvention in which columns of scintillator materials are formed byetching clad materials followed by packing of non-scintillator materialsaround the columns of scintillator materials;

FIG. 8 is a schematic illustration of another embodiment of theinvention using a re-accelerating electrode;

FIG. 9 is a schematic illustration of another embodiment of theinvention using reducing optical fibers;

FIG. 10 is a schematic illustration of another embodiment of theinvention using a large pixel detector; and

FIG. 11 is a variation, in schematic form, of the embodiment illustratedin FIGS. 7A-7C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described with reference to the drawingfigures and to typical uses thereof in which an imaging device such as acharge-coupled device (CCD) camera 20 is mounted on the projectionchamber 10 of a transmission electron microscope (TEM). However, it willbe apparent to those skilled in this art that other constructions andarrangements may be utilized within the scope of the present inventionincluding operation of a TEM in a scanning mode (STEM) and operation ofa scanning electron microscope (SEM).

Typically, the projection chamber is attached to the end of an opticalcolumn of a TEM and houses a viewing screen 12 which is either loweredinto an observation position or raised into a position in which it doesnot intercept electron beam 11 which is projected into the chamber. Theprojection chamber may also house a film magazine comprising a transportmechanism (not shown) which inserts a sheet of photographic film 13 intoan exposure position and returns the sheet into the magazine afterexposure.

The typical projection chamber further has several ports suitable forattaching an imaging device such as a camera, one of which is usuallysituated at the bottom of the chamber. The chamber is normally evacuatedvia a vacuum pipe 14 leading to a gate valve 15 which can either open orclose the chamber to a high vacuum (e.g., 10⁻⁶ torr) pump 18. The gatevalve in most modem TEMs is controlled pneumatically via two inlets 16and 17 such that introduction of pressurized air into one inlet causesthe valve to open, and the introduction of pressurized air into theother inlet causes the valve to close.

An electron beam 11 forming an electron image or diffraction patternfrom a specimen in the microscope traverses the projection chamber 10.Camera 20 includes a scintillator 22 which converts the electron imageinto a light image. Scintillator 22 is supported on a transfer opticsuch as fiber optic plate 24. By light image, it is generally meantlight in the visible spectrum, although there are some scintillationmaterials which can produce light outside of the visible spectrum ineither the near infrared or in the ultraviolet regions of the spectrum.It is within the scope of the present invention to use scintillatormaterials which produce images in the infrared, visible, and/orultraviolet portion of the spectrum.

Fiber optic plate 24 is optically coupled to a an imaging sensor such asa two-dimensional charge-coupled device (CCD) sensor 26 with anoptically-coupling oil layer 46. Such CCD sensors are commerciallyavailable from several manufacturers including Kodak, Ford, ScientificImaging Technologies (SITe), Hamamatsu, Thomson CSF, and EnglishElectric Valve Ltd. Preferred solid-state imaging devices are scientificgrade CCDs whose imaging areas comprise 1024×1024 or more pixels.However, it should be appreciated that any imaging device which iscapable of capturing a light image and producing an electronic signalmay be utilized including a cathode ray television tube.

The preferred CCD must be operated cold to keep its dark current smallenough so that the noise in the dark current accumulated during atypical exposure does not limit the performance of the camera. Thetypical exposure in an electron microscope is from about 1 to 20seconds. Maintaining the CCD at a temperature of about −25° to about−40° C. is typically sufficiently low for the accumulated dark currentto be acceptably small at exposure times of up to about 1 minute. Such atemperature is conventionally achieved using a thermoelectric coolingdevice (not shown), whose cold side may be in contact with the imagingsensor 26.

The CCD is connected to an external electronics unit 30 through a vacuumfeed-through 28 which transfers the captured images to the memory of adigital computer 32. The images may be displayed on a view screen 34,such as a CRT, attached to the computer. For example, the images may bedigitized with 14 bit dynamic range at 450 kHz and then displayed by aPower Macintosh computer using Digital/Micrograph software commerciallyavailable from Gatan, Inc., Pleasanton, Calif. Other details ofoperation are set forth in commonly-owned U.S. Pat. No. 5,065,029, thedisclosure of which is incorporated by reference.

By arranging an imaging electron detector so that it is at a differentelectrical potential than the specimen, the interaction of the electronswith both the specimen and the detector may be controlled to optimizeeach interaction. Different possible arrangements are illustratedschematically in FIGS. 2-4. In each of the embodiments which areillustrated, an electric field generator is positioned adjacent to theimaging detector to adjust the energies of the electrons. In FIG. 2, ina transmission electron microscope (TEM), an electron accelerator ispositioned between a source of electrons and the specimen to provideoptimum interaction at the specimen. An electron decelerator (or insuitable cases, a second accelerator) is positioned immediately abovethe detector/camera to optimize electron energies as they impact thescintillator material on the imaging detector.

A possible arrangement for a transmission electron microscope operatingin a scanning mode is shown in FIG. 3. Again, an electron decelerator(or in suitable cases an accelerator) is positioned immediately abovethe detector/camera to optimize electron energies as they impact thescintillator material on the imaging detector. The present invention isalso applicable to scanning electron microscopes (SEM). As shown in FIG.4, an electron accelerator (or in suitable cases a decelerator) ispositioned immediately above the detector/camera to optimize electronenergies as they impact the scintillator material on the imagingdetector.

In these embodiments of the invention, the electrons are decelerated (orsuitable accelerated) before they interact with the scintillatormaterial on the imaging detector. Where a decelerating electric field isused, this is accomplished by positioning an electrostatic electrondecelerator immediately above the detector to reduce the kinetic energyof the incoming electrons. By reducing the energies of the incomingelectrons, their path lengths in the detector are reduced. This isbecause a lower energy electron may be completely decelerated within theenergy converting volume of the detector. This also reduces the energyconverting volume and increases the efficiency of energy conversion forvirtually all known detectors.

At lower electron energies, such as for example, between about 20 toabout 200 Kev, detector thickness may be optimized to maximize theconversion of energy from the electrons without compromising resolutionof any image which is detected. Additionally, the problem of electronback scatter is significantly reduced because the electrons lose most oftheir energy while in the detector.

By positioning a variable voltage electrostatic decelerator immediatelyabove the detector, one may select a voltage (and thus a deceleration ofincoming electrons) which optimizes electron/detector interactionindependent of the microscope acceleration voltage which was applied.Thus, microscope acceleration voltage may be chosen to optimize theinteraction of the electrons with the sample to be imaged. Currentpractice, in which the electrons are accelerated from, for example anelectron gun, must compromise between electron/specimen interaction andelectron/detector interaction.

In another variation of the invention, an electrostatic acceleratingvoltage may be positioned immediately above or beneath the energyconverting volume of the detector to accelerate emerging electronsthrough and away from the detector. This variation results in a decreasein the extent of electron back scatter and noise in the detector,improving resolution. One implementation of such an electronaccelerating field is shown in FIG. 8 in which such a field is formed bydepositing a thin conductive layer 80 onto the sides of a first fiberoptic 82 located immediately beneath scintillator 84. Such an electrodemay be formed, for example, by deposition of a layer of indium tin oxideand then connecting the electrode to a suitable power source. Theelectron beam 86 (with interaction volume 87 shown), after passingthrough scintillator 84 is accelerated away from the scintillator toreduce back-scattering.

In an alternative embodiment shown in FIG. 5, the energetic electronsare decelerated by positioning a non-scintillating material 52 betweenthe electron beam 54 (with interaction volume 55 depicted) and thescintillating material 56. A fiber optic 58 directs the light to asuitable detector (not shown). Traversing the non-scintillator materialwould reduce the kinetic energies of the electrons prior to theirimpinging upon the scintillator. The type of material selected and itsthickness are selected to optimize the deceleration and spread of theelectron beam. However, one disadvantage of this particular embodimentof the invention is that the non-scintillator material would tend toincrease the angular spread of electron paths prior to reaching thedetector.

In another version of this embodiment, the detector is fabricated usinghigh Z materials (i.e., materials having an average atomic numberdensity which is greater than the majority of materials). For example,cesium iodide is both a high Z material as well as a scintillator. Thus,in this embodiment, the kinetic energies of the electrons are reduced asthe electrons interact with the volume of the detector to a greaterextent than with conventional detector/scintillator materials.

In yet another embodiment of the invention, image resolution is improvedand noise reduced by limiting the lateral spatial travel of theenergetic electrons as they traverse the volume of the detector. Suchlateral spatial travel may be reduced through the use of areas orcolumns of detector material oriented substantially parallel to the pathof the incoming electrons, such columns being substantially surroundedby high Z material. The high Z material acts as a barrier to lateraltravel of the electrons.

One implementation of this embodiment of the invention is illustrated inFIG. 5 in which areas of scintillator materials 60 are contained betweenwalls of high Z material 62 on an imaging detector 64.

Another implementation of this embodiment of the invention can utilizeoptical fibers made with scintillator material and orientedsubstantially parallel to the incoming electrons and surrounded by anelectron decelerating barrier of a high Z material. In the embodimentshown in FIGS. 7A-7C, scintillator material 70 forms the core of opticalfibers 72. The optical fibers are then selectively etched, as shown inFIG. 7B, to expose the scintillator material 70. Finally, as shown inFIG. 7C, the areas around the scintillator material are then packed witha high Z material 74.

In the embodiment of the invention shown in FIGS. 7A-7C, the high Zbarrier materials may also be aligned with the boundaries of the spatialelements (pixels) of the imaging detector. In this manner, light fromthe scintillator material is directed to individual pixels on theimaging detector. For example, the imaging detector may be a CCD asillustrated and described above. In such a case, the barriers arecarefully aligned with the pixels of the CCD. Alternatively, the imagingdetector may be an amorphous silicon detector in which the scintillatorand barrier materials have been deposited directly onto the surface ofthe silicon detector.

In a variation on the embodiment shown in FIGS. 7A-7C, in FIG. 11 anon-scintillating electron deceleration layer 110 may be superimposedover layer 112 of scintillator material atop fiber optic 114. Lateraltravel of the electrons is reduced by forming deceleration layer 110with substantially vertically-oriented columns of a low Z material 116(through which the electrons will pass, but give up energy) surroundedby areas of high Z barrier material 118.

As discussed above, noise from electron back scatter reentering thedetector may be reduced by positioning an electrostatic acceleratingvoltage beneath the energy conversion volume of the detector. Noise andback scatter may also be reduced by utilizing a low Z material (i.e., amaterial having an average atomic number density which is less than themajority of materials) for the support structure of the of thescintillator or the transfer optic may be fabricated of a low Zmaterial. A low Z material will tend not to cause significantinteractions with the electrons so that fewer electrons will be backscattered toward the detector.

In still another embodiment of the invention, resolution is improved andnoise reduced by selecting a pixel size for the measuring and/or storingportion of the imaging detector such that the pixel size encompassessubstantially the entire interaction volume for a majority of theenergetic electrons in the detector (which have evenly distributedpoints of entry). The total converted energy of an electron whichdecelerates completely within the energy conversion volume is constant.If all or substantially all of this energy is accumulated within asingle spatial bin or pixel, noise is greatly reduced. Binning is atechnique whereby the energy detected by groups of adjacent pixels on animaging detector are summed together. In this embodiment, the pixel sizefor the imaging detector must be chosen to be large enough so that amajority of electrons will have expected interaction volumes which willfall within that size, with only a minority of electron havinginteraction volumes which will cross pixel boundaries.

This can be accomplished, for example, as shown in FIG. 9 by using areducing fiber optic 90 such that the electron 92 (with interactionvolume 93) impinging on scintillator 94 is contained within a singlepixel 96 of an imaging detector 98. Alternatively, as shown in FIG. 10,a detector 100 having a large pixel size for individual pixels 102 isused such that electron 104 (with interaction volume 105) passingthrough scintillator 106 and fiber optic 108 is contained within asingle pixel of the detector.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the methods and apparatusdisclosed herein may be made without departing from the scope of theinvention. For example, combinations of the above techniques may also beutilized and be within the scope of this invention.

What is claimed is:
 1. A method for improving resolution and reducingnoise in an image detector for an electron microscope comprising:providing a beam of energetic electrons and an imaging electrondetector; and limiting the lateral spatial travel of said energeticelectrons as they traverse said imaging electron detector.
 2. A methodfor improving resolution and reducing noise in an image detector for anelectron microscope comprising: providing a beam of energetic electronsand an imaging electron detector which includes a layer of ascintillator material; and limiting the lateral spatial travel of saidenergetic electrons as they traverse said imaging electron detector byorienting areas of said scintillator material substantially parallel tothe path of said energetic electrons and at least partially surroundingsaid areas of scintillator material with a material which is a barrierto said energetic electrons.
 3. A method as claimed in claim 2 in whichthe barrier material comprises a high Z material.
 4. A method as claimedin claim 2 in which said scintillator material is contained betweenwalls of the barrier material.
 5. A method as claimed in claim 2including the step of aligning the barrier material with the boundariesof the spatial elements of said imaging detector.
 6. A method as claimedin claim 2 in which said imaging detector comprises an amorphous silicondetector and in which said scintillator and barrier materials aredeposited onto the surface of said amorphous silicon detector.
 7. Amethod for improving resolution and reducing noise in an image detectorfor an electron microscope comprising: providing a beam of energeticelectrons and an imaging electron detector which includes a layer of ascintillator material; and limiting the lateral spatial travel of saidenergetic electrons as they traverse said imaging electron detector bypositioning a non-scintillating electron barrier layer between said beamof energetic electrons and said imaging detector, said non-scintillatinglayer comprising areas of a material through which said energeticelectrons will pass positioned substantially parallel to the path ofsaid energetic electrons and at least partially surrounding said areaswith a material which is a barrier to said energetic electrons.
 8. Amethod for reducing noise in an image detector for an electronmicroscope comprising: providing a beam of energetic electrons and animaging electron detector; and limiting the extent of electron backscatter from said beam of energetic electrons into said imaging electrondetector.
 9. A method as claimed in claim 8 in which the extent ofelectron back scatter is limited by positioning an accelerating electricfield between said energetic electrons and said imaging detector.
 10. Amethod as claimed in claim 8 in which the extent of electron backscatter is limited by positioning an accelerating electric field on theside of said imaging detector opposite the side on which said energeticelectrons impinge.
 11. A method as claimed in claim 8 in which saidimaging detector includes a layer of a scintillator material and theextent of electron back scatter is limited by supporting said layer ofscintillator material with a low Z material.
 12. An apparatus forimproving resolution and reducing noise in an imaging electron detectorfor an electron microscope comprising: a source of a beam of energeticelectrons; an imaging electron detector; and an electric field generatorpositioned adjacent said imaging detector for adjusting the energies ofsaid energetic electrons.
 13. An apparatus as claimed in claim 12 inwhich said electric field generator comprises an electron decelerator.14. An apparatus as claimed in claim 12 in which said electric fieldgenerator comprises an electron accelerator.
 15. An apparatus as claimedin claim 12 in which said electric field generator comprises a variablevoltage electrostatic field generator.
 16. An apparatus as claimed inclaim 12 in which said imaging detector includes a layer of ascintillator material and said electric field generator comprises afiber optic positioned on the side of said scintillator materialopposite that which said energetic electrons impinge, said fiber opticincluding a conductive coating on either edge thereof to formelectrodes, said electrodes communicating with a source of electricpower.
 17. An apparatus for improving resolution and reducing noise inan image detector for an electron microscope comprising: a source of abeam of energetic electrons; an imaging electron detector; and means forlimiting the lateral spatial travel of said energetic electrons as theytraverse said imaging electron detector.
 18. An apparatus for improvingresolution and reducing noise in an image detector for an electronmicroscope comprising: a source of a beam of energetic electrons; animaging electron detector including a layer of scintillator material;and means for limiting the lateral spatial travel of said energeticelectrons as they traverse said imaging electron detector, said meansfor limiting lateral spatial travel comprise areas of said scintillatormaterial oriented substantially parallel to the path of said energeticelectrons, said areas at least partially surrounded by a material whichis a barrier to said energetic electrons.
 19. An apparatus as claimed inclaim 18 in which the barrier material comprises a high Z material. 20.An apparatus as claimed in claim 18 in which said scintillator materialis contained between walls of the barrier material.
 21. An apparatus asclaimed in claim 18 in which the barrier material is aligned with theboundaries of the spatial elements of said imaging detector.